Regret returning to follow former chemist (7)
I believe the answer is:
pasteur
'chemist' is the definition.
(I know that Pasteur is a chemist)
'regret returning to follow former' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'returning' says the letters should be written backwards.
'to follow' means one lot of letters go next to another (some letters go after or follow others).
'former' becomes 'past' (similar in meaning).
'rue' backwards is 'eur'.
'eur' put after 'past' is 'PASTEUR'.
(Other definitions for pasteur that I've seen before include "Louis ---, Fr. bacteriologist" , "Louis --, French bacteriologist" , "Louis -, nineteenth century French chemist" , "French chemist and biologist, d. 1895" , "Pioneer immunologist" .)